Mixed powder for powder metallurgy, method of manufacturing same, and method of manufacturing iron-based powder sintered body

ABSTRACT

A mixed powder for powder metallurgy includes a machinability improvement powder that is crystalline layered alkaline silicate heat-treated at 400° C. to 1100° C. and whose mix proportion is in an amount of 0.01% to 1.0% by mass in terms of total content of an iron-based powder, an alloying powder, and the machinability improvement powder. Such a mixed powder not only enables a compact to be sintered without adversely affecting the environment in a sintering furnace, but also enables a sintered body having excellent lathe machinability and excellent drill machinability to be obtained.

TECHNICAL FIELD

The disclosure relates to a mixed powder for powder metallurgy obtained by mixing an iron-based powder, an alloying powder, a machinability improvement powder, and a lubricant and suitable for sintered parts of vehicles and the like and a method of manufacturing the same, and a method of manufacturing an iron-based powder-made sintered body by forming and sintering the mixed powder. The disclosure is particularly intended to improve the machinability of an iron-based powder-made sintered body.

BACKGROUND

The development of powder metallurgy technology has enabled parts with high dimensional accuracy and complex shape to be manufactured in near net shape. Products made using powder metallurgy technology are thus utilized in various fields. Powder metallurgy technology has high shape flexibility, as a die of the desired shape is filled with a powder which is then formed and sintered. Hence, powder metallurgy technology is often used for machine parts having complex shapes such as gears.

In the field of iron-based powder metallurgy, a die of a predetermined shape is filled with an iron-based mixed powder obtained by mixing an iron-based powder (metal powder) with an alloying powder such as a copper powder or a graphite powder and a lubricant such as zinc stearate or lithium stearate, which is then press-formed into a compact and subjected to a sintering process to obtain a sintered part. The sintered part obtained in this way typically has high dimensional accuracy. In manufacturing a sintered part for which extremely strict dimensional accuracy is required, cutting work needs to be performed after sintering. The cutting work includes processes such as lathe turning and drill boring at various cutting speeds.

The sintered part has high porosity, and so has a high cutting resistance as compared to a metal material processed by melting. Accordingly, to improve the machinability of the sintered body, Pb, Se, Te or the like has been conventionally added to the iron-based mixed powder in powder form or in the form of being alloyed with the iron powder or iron-based powder.

However, the use of Pb has a problem in that, since Pb has a low melting point of 330° C., Pb melts in the sintering process, but does not dissolve in iron. So, it is difficult to uniformly disperse Pb in the matrix. The use of Se or Te has a problem in that the sintered body is embrittled and as a result the mechanical property of the sintered body degrades significantly.

Besides, due to poor thermal conductivity of the aforementioned pores, when the sintered body is cut, frictional heat during the cutting accumulates and as a result the surface temperature of the tool tends to increase. The cutting tool thus wears easily and has a shorter life. This leads to the problem of an increase in cutting work cost and an increase in manufacturing cost of sintered parts.

In view of these problems, for example, JP S61-147801 A describes an iron powder mixture for sintered body production obtained by mixing an iron powder with 0.05% to 5% by weight a fine manganese sulfide powder of 10 μm or less.

The technique described in JP S61-147801 A is supposed to improve the machinability of the sintered material without significant dimensional changes and strength deterioration.

JP S60-145353 A describes an iron-based sintered body manufacturing method of adding alkaline silicate to an iron-based powder.

The technique described in JP S60-145353 A is supposed to improve free machinability without significant dimensional changes and strength deterioration, by adding 0.1% to 1.0% by weight alkaline silicate.

JP H9-279204 A describes an iron-based mixed powder for powder metallurgy mainly composed of an iron powder and contains 0.02% to 0.3% by weight a CaO—Al₂O₃—SiO₂ complex oxide powder (ceramic powder) of 50 μm or less in average particle size having an anorthite phase and/or a gehlenite phase.

The technique described in JP H9-279204 A is supposed to prevent tool material degradation and improve machinability, as the ceramic powder exposed on the worked surface adheres to the tool surface and forms a tool protective film (belag layer) during cutting.

JP 2006-89829 A describes an iron-based mixed powder obtained by mixing an iron-based powder, an alloying powder, a machinability improvement powder including a manganese sulfide powder and at least one of a calcium phosphate powder and a hydroxyapatite powder, and a lubricant. According to JP 2006-89829 A, the manganese sulfide is effective in refinement of chips, whereas the calcium phosphate powder and the hydroxyapatite powder have an effect of preventing or suppressing tool surface alteration by adhering to the tool surface and forming a belag layer during cutting.

The technique described in JP 2006-89829 A is thus supposed to improve machinability without degradation of the mechanical property of the sintered body.

JP S46-39564 B describes an improvement in mechanical workability such as machinability by adding, to iron or an iron-based alloy, 0.3% to 3.0% by weight barium sulfate, barium sulfide, or both.

However, the techniques described in JP S61-147801 A and JP 2006-89829 A have a problem in that, since a manganese sulfide (MnS) powder is contained, the appearance of the sintered body deteriorates, and also S or MnS remaining in the sintered body promotes rusting of the sintered part and lowers its anti-corrosion property.

These techniques also have a problem in that, though MnS is excellent in improving machinability in a low speed range where the cutting speed is 100 m/min or less, the machinability improvement effect of MnS is small in high speed cutting of about 200 m/min.

The technique described in JP S60-145353 A has a problem in that, since alkaline silicate is hygroscopic, fixation occurs in the mixed powder and causes a forming failure.

The technique described in JP H9-279204 A has a problem in that impurities in the ceramic powder need to be reduced and also the particle size needs to be adjusted to prevent decreases in powder property and sintered body property, which incurs a rising material cost. The technique described in JP H9-279204 A also has a problem in that, though excellent in improving machinability at high speed, the machinability improvement effect is small in low speed cutting.

The machinability improvement by belag layer formation described in JP H9-279204 A and JP 2006-89829 A is effective in reducing cutting power in lathe turning, but has poor chip removability in drilling as chips are not refined. Thus, there is still a problem with drill machinability.

The technique described in JP S46-39564 B has a problem in that the machinability improvement effect is small in high speed cutting of about 200 m/min, as in using MnS.

It could therefore be helpful to provide a mixed powder for powder metallurgy that enables obtainment of a sintered body having excellent machinability and particularly excellent lathe machinability (hereafter also referred to as lathe turnability) and excellent drill machinability, and a method of manufacturing the same. It could also be helpful to provide a method of manufacturing an iron-based powder-made sintered body having excellent machinability including both excellent lathe turnability and excellent drill workability.

SUMMARY

We thus provide:

1. A mixed powder for powder metallurgy obtained by mixing an iron-based powder, an alloying powder, a machinability improvement powder, and a lubricant, wherein the machinability improvement powder comprises crystalline layered alkaline silicate heat-treated at 400° C. to 1100° C., and a mix proportion of the machinability improvement powder is 0.01% to 1.0% by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder.

2. The mixed powder for powder metallurgy according to the foregoing 1, wherein the machinability improvement powder further comprises at least one selected from a group consisting of an enstatite powder, a talc powder, a kaolin powder, a mica powder, a granulated slag powder, a levigated clay powder, a magnesium oxide (MgO) powder, and a powder mixture of silica (SiO₂) and magnesium oxide (MgO), in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.

3. The mixed powder for powder metallurgy according to the foregoing 2, wherein the machinability improvement powder further comprises an alkali metal salt powder in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.

4. The mixed powder for powder metallurgy according to the foregoing 3, wherein the alkali metal salt powder is one or two selected from a group consisting of an alkali carbonate powder and an alkali metal soap.

5. The mixed powder for powder metallurgy according to any one of the foregoing 1 to 4, wherein the machinability improvement powder further comprises a calcium fluoride powder.

6. The mixed powder for powder metallurgy according to any one of the foregoing 1 to 5, wherein the machinability improvement powder further comprises one or two selected from a group consisting of a metal boride powder and a metal nitride powder.

7. The mixed powder for powder metallurgy according to the foregoing 6, wherein the metal boride powder consists of at least one selected from a group consisting of TiB₂, ZrB₂, and NbB₂, and the metal nitride powder consists of at least one selected from a group consisting of TiN, AlN, and Si₃N₄.

8. The mixed powder for powder metallurgy according to any one of the foregoing 1 to 7, wherein the machinability improvement powder further comprises at least one selected from a group consisting of an alkali metal sulfate and an alkaline earth metal sulfate, in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.

9. A method of manufacturing a mixed powder for powder metallurgy according to any one of the foregoing 1 to 8, by preparing and then mixing an iron-based powder, an alloying powder, a machinability improvement powder, and a lubricant to obtain a mixed powder, wherein the machinability improvement powder comprises crystalline layered alkaline silicate heat-treated at 400° C. to 1100° C., and a mix proportion of the machinability improvement powder is 0.01% to 1.0% by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder, and the mixing includes: primary mixing in which a part or whole of the machinability improvement powder and a part of the lubricant are added, as a primary mixture material, to the iron-based powder and the alloying powder and heated to perform mixing while melting at least one type of the lubricant, and a resulting mixture is cooled for solidification; and secondary mixing in which a remaining powder of the machinability improvement powder and the lubricant is added, as a secondary mixture material, to the mixture to perform mixing.

10. The method of manufacturing a mixed powder for powder metallurgy according to the foregoing 9, wherein the machinability improvement powder further comprises at least one selected from a group consisting of an enstatite powder, a talc powder, a kaolin powder, a mica powder, a granulated slag powder, a levigated clay powder, a magnesium oxide (MgO) powder, and a powder mixture of silica (SiO₂) and magnesium oxide (MgO), in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.

11. The method of manufacturing a mixed powder for powder metallurgy according to the foregoing 10, wherein the machinability improvement powder further comprises an alkali metal salt powder in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.

12. The method of manufacturing a mixed powder for powder metallurgy according to the foregoing 11, wherein the alkali metal salt powder is one or two selected from a group consisting of an alkali carbonate powder and an alkali metal soap.

13. The method of manufacturing a mixed powder for powder metallurgy according to any one of the foregoing 9 to 12, wherein the machinability improvement powder further comprises a calcium fluoride powder.

14. The method of manufacturing a mixed powder for powder metallurgy according to any one of the foregoing 9 to 13, wherein the machinability improvement powder further comprises one or two selected from a group consisting of a metal boride powder and a metal nitride powder.

15. The method of manufacturing a mixed powder for powder metallurgy according to the foregoing 14, wherein the metal boride powder consists of at least one selected from a group consisting of TiB₂, ZrB₂, and NbB₂, and the metal nitride powder consists of at least one selected from a group consisting of TiN, AlN, and Si₃N₄.

16. The method of manufacturing a mixed powder for powder metallurgy according to any one of the foregoing 9 to 15, wherein the machinability improvement powder further comprises at least one selected from a group consisting of an alkali metal sulfate and an alkaline earth metal sulfate, in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.

17. A method of manufacturing an iron-based powder sintered body, by filling a die with a mixed powder for powder metallurgy manufactured by the method according to any one of the foregoing 9 to 16, compression-forming the mixed powder into a compact, and subjecting the compact to a sintering process to obtain a sintered body.

It is thus possible to manufacture a sintered body having excellent machinability that includes both excellent lathe turnability and excellent drill machinability, at low cost. This remarkably reduces the manufacturing cost of metal sintered parts, and has an industrially significant advantageous effect. Since cutting can be performed in a wide range of cutting conditions from low speed to high speed, the advantageous effect is particularly noticeable in works such as drilling, where the cutting speed varies between center and peripheral portions.

Another advantageous effect is that a compact can be formed without a decrease in green density and an increase in ejection force.

DETAILED DESCRIPTION

We investigated various factors, especially alkaline silicate, affecting the machinability of the sintered body. As a result of conducting a test of high-temperature heat treatment to reduce the hygroscopicity of alkaline silicate, we discovered that alkaline silicate crystallized in layers by the heat treatment significantly improves the machinability of the sintered body.

The mechanism of this improvement is still unclear. However, JP H04-157138 A, for example, describes that a magnesium metasilicate-based mineral or a magnesium orthosilicate-based mineral is cleavable and functions as a solid lubricant and, as a result, improves the free machinability, slidability, conformability, and wear resistance of the alloy. We assume that crystalline layered alkaline silicate has the same mechanism.

We also discovered that crystalline layered alkaline silicate has a greater machinability improvement effect than a magnesium metasilicate-based mineral or magnesium orthosilicate, and is effective in machinability improvement even at relatively low speed, that is, effective in machinability improvement in a wide range from low speed to high speed.

The mechanism of this improvement is still unclear. However, given that MnS and the like have been reported to have an action of fostering a ductile fracture of a shear zone under low strain shear rate deformation, the same mechanism is believed to function more advantageously.

We thus determined that crystalline layered alkaline silicate can simultaneously improve machinability of different requirements, namely, machinability by a lathe (lathe turnability) and machinability by a drill (drill machinability).

We also discovered that lathe turnability at low speed can be further improved by adding, as a machinability improvement powder (additive), not only crystalline layered alkaline silicate but also a powder including at least one selected from a group consisting of SiO₂ and MgO.

The mechanism of the synergistic machinability improvement of the sintered body is still unclear, but we assume the following.

According to the description of JP 2012-144801 A, the addition of a powder including one selected from a group consisting of SiO₂ and MgO allows a soft phase and a hard phase to be simultaneously dispersed in the matrix phase of the sintered body during the sintering process. Therefore, when a powder including one selected from a group consisting of SiO₂ and MgO is added to crystalline layered alkaline silicate, the function of the crystalline layered alkaline silicate as a solid lubricant becomes more apparent, and decreases the drag of the soft metal compound phase exerted on the tool. This facilitates the function of suppressing wear, deformation, or cracking of the tool, and facilitates cracking in chips by the hard metal compound phase, contributing to enhanced removability of chips during drill boring.

We thus discovered that adding crystalline layered alkaline silicate to the additive described in JP 2012-144801 A produces the synergistic effect of machinability improvement in drilling.

We further discovered that lathe turnability at low speed can be further improved by adding, as a machinability improvement powder (additive), not only crystalline layered alkaline silicate but also a powder including at least one selected from a group consisting of alkali metal sulfates and alkaline earth metal sulfates.

The mechanism of the synergistic machinability improvement of the sintered body is still unclear, but we assume the following.

According to the description of JP S46-39564 B, BaSO₄ does not melt or dissolve in any metal and is soft, and such BaSO₄ scatters in crystal grain boundaries and grains and develops a notch effect during cutting, thus lowering the cutting resistance and improving the machinability by cutting.

Therefore, when a powder including at least one selected from a group consisting of alkali metal sulfates and alkaline earth metal sulfates is added to crystalline layered alkaline silicate, the function of the crystalline layered alkaline silicate as a solid lubricant becomes more apparent, and decreases the drag of the soft compound phase exerted on the tool. This further enhances the function of suppressing wear, deformation, or cracking of the tool.

We thus discovered that adding crystalline layered alkaline silicate to the additive described in JP S46-39564 B produces the synergistic effect of machinability improvement in drilling and the like at low speed.

The disclosed components and methods are described in detail below. The disclosed mixed powder for powder metallurgy is described first. The disclosed mixed powder for powder metallurgy is a mixed powder obtained by mixing an iron-based powder, an alloying powder, a machinability improvement powder, and a lubricant.

The iron-based powder may be any of the iron-based powders including: a pure iron powder such as an atomized iron powder or a reduced iron powder; a pre-alloyed steel powder (completely alloyed steel powder) obtained by pre-alloying an alloying element; a partial diffusion-alloyed steel powder obtained by partially diffusing and alloying an alloying element in an iron powder; and a hybrid steel powder obtained by further partially diffusing an alloying element in a pre-alloyed steel powder (completely alloyed steel powder). As the iron-based powder, an iron-based powder mixture including an alloying powder and a lubricant in addition to the aforementioned iron-based powder may be used.

The alloying powder is, for example, a graphite powder, a non-ferrous metal powder such as Cu (copper) powder, Mo powder, or Ni powder, a cuprous oxide powder or the like. The alloying powder is selected from these powders and mixed depending on the desired sintered body property. Mixing such an alloying powder with the iron-based powder increases the strength of the sintered body, and ensures the desired sintered part strength. The mix proportion of the alloying powder is 0.1% to 10% by mass in terms of total content of the metal powder, the alloying powder, and the machinability improvement powder, depending on the desired sintered body strength. When the mixed proportion of the alloying powder is less than 0.1% by mass, the desired sintered body strength cannot be ensured. When the mixed proportion of the alloying powder exceeds 10% by mass, the dimensional accuracy of the sintered body decreases.

The machinability improvement powder is crystalline layered alkaline silicate heat-treated at 400° C. to 1100° C. Alkaline silicate used here may be sodium silicate, potassium silicate, lithium silicate or the like. These substances are water-soluble. Accordingly, when any of these substances is directly added to the mixed powder, its moisture absorption causes fixation between the powders in the mixed powder, as a result of which the fluidity of the powder deteriorates and a forming failure occurs.

In view of this, the alkaline silicate is heat-treated to reduce silanol groups on the surface, thus lowering the connectivity with water. It is important to set the heating temperature to 400° C. to 1100° C. When the heating temperature is less than 400° C., the hygroscopicity reduction effect is insufficient. When the heating temperature exceeds 1100° C., the cost of the treatment is not reasonable.

In the heat treatment, the alkaline silicate crystallizes into a layered structure. This structure can be observed by analysis means such as an X-ray diffractometer. The crystalline layered alkaline silicate is one type of crystalline alkali metal layered silicate. The crystalline alkali metal layered silicate is well known as a detergent builder which is a material that, when mixed in a detergent, significantly enhances detergency. The crystalline alkali metal layered silicate is described in detail in JP 2001-114509 A.

When forming the mixed powder into the compact and sintering it, a soft metal compound powder is preferably added, as a machinability improvement powder used together with the crystalline layered alkaline silicate. The soft metal compound powder forms, in the matrix phase of the sintered body, soft particles (soft phase) with lower hardness than the average hardness of the matrix phase and can form an amorphous phase at a low temperature because of the low melting point.

In detail, the soft metal compound powder is at least one type selected from an enstatite powder, a talc powder, a kaolin powder, a mica powder, a granulated slag powder, a levigated clay powder, a magnesium oxide (MgO) powder, and a powder mixture of silica (SiO₂) and magnesium oxide (MgO).

Of these additives to the mixed powder as a machinability improvement powder, soft minerals such as an enstatite powder, a talc powder, a kaolin powder, and a mica powder are metal compounds containing at least Si or Mg, and O (SiO₂ or MgO), and a granulated slag powder is a deoxidation product represented by a chemical composition such as CaO—SiO₂—Al₂O₃ or MgO—Al₂O₃—SiO₂. These powders which are compounds containing Si, Mg, and O can each form a low-melting amorphous phase and disperse in the matrix phase of the sintered body as a soft metal compound phase, when sintering the green compact formed from the mixed powder. The low-melting amorphous phase formed during sintering is a SiO₂—MgO-based amorphous phase.

At least one selected from a group consisting of a levigated clay powder, a magnesium oxide (MgO) powder, and a powder mixture of silica (SiO₂) and magnesium oxide (MgO) which contains Si, Mg, and O as with an enstatite powder and the like may be used as a machinability improvement powder. The powder mixture of silica (SiO₂) and magnesium oxide (MgO) can equally form a low-melting amorphous phase (amorphous particles) when sintering the green compact formed from the mixed powder. The mixing ratio SiO₂:MgO is preferably 1:2 to 3:1 by mass.

Preferably, an alkali metal salt powder is further added as a machinability improvement powder. Further adding an alkali metal salt powder to a powder containing SiO₂ and/or MgO such as an enstatite powder, facilitates the formation of the low-melting amorphous phase when sintering the green compact.

During sintering, not only the alkali metal salt forms low-melting flux by itself or by reacting with iron oxide on the surface of the iron-based powder, but also other oxides such as SiO₂ and MgO included in the mixed powder melt in the flux to form a SiO₂—MgO-alkali metal oxide-based amorphous phase, which disperses in the matrix phase of the sintered body as a soft phase.

Examples of the alkali metal salt include alkali carbonate and alkali metal soap. Any one or a composite of these powders may be included. The use of alkali metal soap is advantageous in that the lubrication effect by the metal soap improves the density of the green compact in powder forming.

The mix proportion of the powder containing SiO₂ and/or MgO or the alkali metal salt powder is preferably 10% to 80% by mass in terms of total content of the machinability improvement powder. When the mix proportion is less than 10% by mass, the aforementioned synergetic effect cannot be expected. When the mix proportion exceeds 80% by mass, the machinability improvement effect at low speed decreases.

A calcium fluoride powder may further be included. The mix proportion of the calcium fluoride powder is preferably 20% to 80% by mass in terms of total content of the machinability improvement powder. When the mix proportion is less than 20% by mass, the desired machinability improvement effect cannot be expected. When the mix proportion exceeds 80% by mass, the mechanical strength of the sintered body decreases.

A powder serving as hard particles is, for example, a metal boride powder and/or a metal nitride powder. Examples of the metal boride powder include a TiB₂ powder, a ZrB₂ powder, and a NbB₂ powder, and a NbB₂ powder is particularly preferable. Examples of the metal nitride powder include a TiN powder, a AlN powder, and a Si₃N₄ powder, and a Si₃N₄ powder is particularly preferable.

The mix proportion of the metal boride powder and/or metal nitride powder is preferably 10% to 80% by mass in terms of total content of the machinability improvement powder. When the mix proportion is less than 10% by mass, the desired machinability improvement effect cannot be expected. When the mix proportion exceeds 80% by mass, the powder compressibility and the sintered body strength decrease.

Moreover, when forming the mixed powder into the compact and sintering it, at least one selected from a group consisting of alkali metal sulfates and alkaline earth metal sulfates may be added as a machinability improvement powder used together with the crystalline layered alkaline silicate.

In detail, at least one selected from a group consisting of alkali metal sulfates such as sodium sulfate and lithium sulfate and alkaline earth metal sulfates such as calcium sulfate, magnesium sulfate, barium sulfate, and strontium sulfate may be added.

These are all soft substances, and do not melt or dissolve in any metal. Such a substance scatters in crystal grain boundaries and grains, and develops a notch effect during cutting, thus lowering the cutting resistance and improving the machinability by cutting. As a result, the function of the crystalline layered alkaline silicate as a solid lubricant becomes more apparent, and decreases the drag of the soft compound phase exerted on the tool. This enhances the function of suppressing wear, deformation, or cracking of the tool.

The mix proportion of the alkali metal sulfate or alkaline earth metal sulfate is preferably 10% to 80% by mass in terms of total content of the machinability improvement powder. When the mix proportion is less than 10% by mass, the desired machinability improvement effect cannot be expected. When the mix proportion exceeds 80% by mass, the powder compressibility and the sintered body strength decrease.

The mix proportion of the machinability improvement powder in the mixed powder needs to be 0.01% to 1.0% by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder. When the mix proportion is less than 0.01% by mass, the machinability improvement effect is insufficient. When the mix proportion exceeds 1.0% by mass, the green density decreases and the mechanical strength of the sintered body obtained by sintering the compact decreases. The mix proportion of the machinability improvement powder in the mixed powder is therefore limited to 0.01% to 1.0% by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder.

The mixed powder includes an appropriate amount of lubricant, in addition to the aforementioned iron-based powder, alloying powder, and machinability improvement powder. The lubricant is preferably metal soap such as zinc stearate or lithium stearate, carboxylic acid such as oleic acid, or amide wax such as stearic acid amide, stearic acid bisamide, or ethylene-bis-stearamide. The mix proportion of the lubricant is not particularly limited. As the external additive amount, the mix proportion is preferably 0.1% to 1.0% by mass in outer percentage in terms of total content of 100% by mass the metal powder, the alloying powder, and the machinability improvement powder. When the mix proportion of the lubricant is less than 0.1% by mass in outer percentage, the friction with the die increases and the ejection force increases, causing a shorter die life. When the mix proportion of the lubricant is large exceeding 1.0% by mass in outer percentage, the forming density decreases and the density of the sintered body decreases.

A preferable method of manufacturing the mixed powder is described below.

The alloying powder, the machinability improvement powder including one or more powders of the aforementioned types and mix proportions, and the lubricant are added to the iron-based powder by respective predetermined amounts. Desirably, they are mixed at one time or in two or more times typically using a well-known mixer to obtain the mixed powder (iron-based mixed powder). The machinability improvement powder does not necessarily need to be mixed all at once. Only a part of the machinability improvement powder may be added and mixed (primary mixing), after which the remaining part (secondary mixture material) is added and mixed (secondary mixing). The lubricant is preferably added twice.

The iron-based powder that has been subjected to segregation prevention treatment of causing a part or whole of the alloying powder and/or machinability improvement powder to adhere to the surface of a part or whole of the iron-based powder by a bonding material may be used. The segregation prevention treatment may be the segregation prevention treatment described in JP 3004800 B.

By heating to not lower than the minimum temperature of the melting point of any of various types of lubricant included in the mixed powder, at least one type of lubricant out of these lubricants melts to initiate primary mixing, and then the mixture is cooled for solidification. After this, the secondary mixture material composed of the remaining powder of the machinability improvement powder and lubricant is added to initiate secondary mixing.

The mixing means is not particularly limited, and may be any of the conventionally well-known mixers. Mixers that facilitate heating such as a high-speed bottom stirring mixer, an inclined rotating pan-type mixer, a rotating hoe-type mixer, and a conical planetary screw-type mixer, are especially advantageous.

A preferable method of manufacturing the sintered body using the mixed powder for powder metallurgy obtained by the aforementioned manufacturing method is described below.

First, a die is filled with the mixed powder for powder metallurgy manufactured by the aforementioned method, which is then compression-formed into a compact. As the forming method, any of the well-known forming methods such as press forming may be suitably used. The use of the disclosed mixed powder for powder metallurgy realizes high forming pressure of 294 MPa or more, and enables forming at normal temperature. To ensure stable formability, it is preferable to heat the mixed powder or the die to appropriate temperature, or apply a lubricant to the die.

In performing compression forming in a heating atmosphere, the temperature of the mixed powder or die is preferably less than 150° C. This is because the mixed powder for powder metallurgy has high compressibility and so exhibits excellent formability even when the temperature is less than 150° C., and also because degradation due to oxidation may occur when the temperature is 150° C. or more.

The compact obtained by the aforementioned forming process is then subjected to the sintering process to form the sintered body. The temperature of the sintering process is desirably about 70% of the melting point of the metal powder.

In the iron-based powder, the temperature of the sintering process is 1000° C. or more, and preferably 1300° C. or less. When the temperature of the sintering process is less than 1000° C., the desired density of the sintered body is unlikely to be achieved. A high temperature of the sintering process exceeding 1300° C. is not preferable because abnormal grain growth tends to occur during sintering and decrease the strength of the sintered body.

The atmosphere of the sintering process is preferably an inert gas atmosphere such as nitrogen or argon, an inert gas-hydrogen gas mixture atmosphere where hydrogen is mixed with the inert gas atmosphere, or a reduction atmosphere such as ammonia decomposition gas, RX gas, or natural gas.

After the sintering process, heat treatment such as gas carburizing heat treatment or carburizing nitriding treatment is further performed according to need, to obtain the product (sintered part or the like) having the desired properties. Cutting work and the like are conducted as necessary to form the product having predetermined dimensions.

Examples

Non-limiting examples according to the disclosure are described below.

As the iron-based powder, the iron-based powders (average particle size: about 80 μm in each case) shown in Table 1 were used. The average particle size was determined using laser diffractometry.

As shown in Table 1, the iron-based powders used are: (A) an atomized pure iron powder; (B) a reduced pure iron powder; (C) a partial diffusion-alloyed steel powder obtained by partially diffusing and alloying Cu as an alloying element on the surface of an iron powder; (D) a partial diffusion-alloyed steel powder obtained by partially diffusing and alloying Ni, Cu, and Mo as an alloying element on the surface of an iron powder; (E) a pre-alloyed steel powder (completely alloyed steel powder) obtained by pre-alloying Ni and Mo as an alloying element; (F) a pre-alloyed steel powder (completely alloyed steel powder) obtained by pre-alloying Mo as an alloying element; (G) a pre-alloyed steel powder (completely alloyed steel powder) obtained by pre-alloying Mo as an alloying element; and (H) a steel powder (hybrid alloyed steel powder) obtained by further partially diffusing and alloying Mo as an alloying element in a completely alloyed steel powder obtained by pre-alloying Mo.

TABLE 1 Iron-based powder Composition symbol Type (%: mass %) A Atomized pure iron powder Fe B Reduced pure iron powder Fe C Partial diffusion-alloyed steel Fe—2.0% Cu powder D Partial diffusion-alloyed steel Fe—4.0% Ni—1.5% powder Cu—0.5% Mo E Completely alloyed steel powder Fe—0.5% Ni—0.5% Mo F Completely alloyed steel powder Fe—0.6% Mo G Completely alloyed steel powder Fe—0.45% Mo H Hybrid alloyed steel powder (Fe—0.45% Mo)—0.15% Mo* *Steel powder obtained by diffusing and alloying 0.15% Mo in Fe—0.45% Mo completely alloyed steel powder

The alloying powder of each of the types and mix proportions shown in Table 2, the machinability improvement powder of each of the types and mix proportions shown in Table 2, and the lubricant of each of the types and mix proportions shown in Table 2 were added to the corresponding one of the aforementioned iron-based powders, and primary mixing was performed using a high-speed bottom stirring mixer. In the primary mixing, each sample was heated to 140° C. while being mixed, and then cooled to 60° C. or less. A natural graphite powder added as the alloying powder is a powder of 5 μm in average particle size, and a copper powder added as the alloying powder is a powder of 20 μm in average particle size.

TABLE 2 Machinability improvement powder Iron-based Primary mixing Lubricant powder Alloying powder Additive group II Secondary Primary mixing Secondary mixing Symbol: Graphite powder Additive group I Proportion of mixing Type*****: Type*****: Mixed mix Type: mix Copper powder Type*: Type**: additive group Type: mix Powder mix proportion mix proportion powder proportion proportion Type: mix proportion mix proportion mix proportion II*** proportion proportion**** (mass %-outer (mass %-outer symbol (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) percentage) percentage) Remarks M 1 A: 99.3 Natural graphite: — a: 0.1 — 0 — 0.1 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.6 M 2 A: 99.2 Natural graphite: — b: 0.1 f: 0.1 50 — 0.2 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.6 M 3 A: 97.1 Natural graphite: Atomized copper: 2.0 a: 0.1 — 0 — 0.1 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.8 M 4 A: 97.0 Natural graphite: Atomized copper: 2.0 c: 0.1 l: 0.05, m: 0.05 20 — 0.2 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Example 0.8 M 5 A: 97.05 Natural graphite: Atomized copper: 2.0 a: 0.05 g: 0.05, h: 0.05 67 — 0.15 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Example 0.8 M 6 B: 99.2 Natural graphite: — b: 0.1 i: 0.05, j: 0.05 50 — 0.2 AO: 0.2, BS: 0.2 Zn: 0.4 Example 0.6 M 7 B: 99.2 Natural graphite: — c: 0.1 k: 0.1 25 — 0.2 AO: 0.2, BS: 0.2 Zn: 0.4 Example 0.6 M 8 B: 96.9 Natural graphite: Electrolyte copper: 2.0 a: 0.1 j: 0.1 33 Si₃N₄: 0.1 0.3 AM: 0.1, BS: 0.1 Zn: 0.1, BS0.5 Example 0.8 M 9 C: 98.9 Natural graphite: — a: 0.1 l: 0.05, m: 0.05 25 TiB₂: 0.2 0.4 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.7 M 10 D: 99.2 Natural graphite: — b: 0.2 n: 0.1 33 — 0.3 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.5 M 11 E: 99.3 Natural graphite: — c: 0.1 o: 0.1 50 — 0.2 AO: 0.2, BS: 0.2 Zn: 0.4 Example 0.5 M 12 F: 99.05 Natural graphite: — a: 0.2 l: 0.1, m: 0.05 43 TiB₂: 0.1 0.45 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Example 0.5 M 13 G: 96.8 Natural graphite: Atomized copper: 2.0 b: 0.2 k: 0.1 25 ZrB₂: 0.1 0.4 AM: 0.1, BS: 0.1 Zn: 0.1, BS0.5 Example 0.8 M 14 H: 98.5 Natural graphite: — c: 0.2 f: 0.25, o: 0.25 71 — 0.7 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.8 M 15 A: 99.4 Natural graphite: — — — 0 — 0 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.6 Example M 16 A: 99.3 Natural graphite: — d: 0.1 — 0 — 0.1 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.6 Example M 17 A: 97.1 Natural graphite: Atomized copper: 2.0 d: 0.1 — 0 — 0.1 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.8 Example M 18 A: 97.1 Natural graphite: Atomized copper: 2.0 e: 0.1 — 0 — 0.1 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.8 Example M 19 A: 97.1 Natural graphite: Atomized copper: 2.0 — f: 0.1 100 — 0.1 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.8 Example M 20 A: 97.1 Natural graphite: Atomized copper: 2.0 — l: 0.05, m: 0.05 25 — 0.1 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Comparative 0.8 Example M 21 B: 97.0 Natural graphite: Electrolyte copper: 2.0 — j: 0.1 50 Si₃N₄: 0.1 0.2 AM: 0.1, BS: 0.1 Zn: 0.1, BS0.5 Comparative 0.8 Example M 22 C: 99.3 Natural graphite: — — — 0 — 0 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.7 Example M 23 D: 99.5 Natural graphite: — — — 0 — 0 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.5 Example M 24 E: 99.5 Natural graphite: — — — 0 — 0 AO: 0.1, Zn: 0.3 Zn: 0.4 Comparative 0.5 Example M 25 F: 99.25 Natural graphite: — — l: 0.1, m: 0.05 60 TiB₂: 0.1 0.25 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Comparative 0.5 Example M 26 G: 97.2 Natural graphite: Atomized copper: 2.0 — — 0 — 0 AM: 0.1, BS: 0.1 Zn: 0.1, BS0.5 Comparative 0.8 Example M 27 A: 99.2 Natural graphite: — a: 0.1 p: 0.1 50 — 0.2 AO: 0.1, Zn: 0.3 Zn: 0.4 Example 0.6 M 28 A: 97.0 Natural graphite: Atomized copper: 2.0 a: 0.1 q: 0.1 20 — 0.2 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Example 0.8 M 29 A: 97.0 Natural graphite: Atomized copper: 2.0 a: 0.1 r: 0.1 20 — 0.2 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Example 0.8 M 30 A: 97.05 Natural graphite: Atomized copper: 2.0 a: 0.05 f: 0.05, p: 0.05 67 — 0.15 AM: 0.2, BS: 0.2 Li: 0.1, Zn0.1, BS0.3 Example 0.8 *a: crystalline layered sodium silicate, b: crystalline layered potassium silicate, c: crystalline layered lithium silicate, d: unheated sodium silicate, e: 350° C. heated sodium silicate **f: enstatite, g: talc, h: kaolin, i: mica, j: granulated slag, k: levigated clay, l: MgO, m: SiO2, n: lithium carbonate (alkali metal salt) o: lithium stearate (metal soap) p: sodium sulfate, q: calcium sulfate, r: barium sulfate ***proportion of additive group II in all cuttability improvement powder ****Proportion of all cuttability improvement powder in total content of iron-based powder, alloying powder, and cuttability improvement powder *****Zn: zinc stearate, Li: lithium stearate, BS: ethylene-bis-stearamide, AM: stearic acid monoamide, AO: oleic acid

After the primary mixing, the secondary mixture material composed of the machinability improvement powder and lubricant of each of the types and mix proportions shown in Table 2 was further added, and secondary mixing of stirring each sample for 1 minute was performed with the rotational frequency of the mixer being 1000 rpm. After the secondary mixing, the mixed powder was taken out of the mixer. The machinability improvement powder was added twice, i.e., upon the primary mixing and upon the secondary mixing. The mix proportion of the machinability improvement powder is expressed in % by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder. The mix proportion of the lubricant as external addition is expressed in % by mass in outer percentage in terms of total content of 100% by mass the iron-based powder, the alloying powder, and the machinability improvement powder.

As a result of the aforementioned steps, the mixed powder in which the iron-based powder, the alloying powder, and the machinability improvement powder were uniformly mixed without segregation was obtained.

As comparative examples, the iron-based powder, the alloying powder, and the lubricant of each of the types and mix proportions shown in Table 2 were added and mixed at normal temperature using a V-type container rotating mixer, thus obtaining a mixed powder.

Following this, a die (two types for lathe turning test and drilling test) was filled with the obtained mixed powder, which was then compression-formed with a pressing force of 590 MPa to obtain a compact. The compact was subjected to a sintering process at 1130° C. for 20 min in an RX gas atmosphere to obtain a sintered body.

The obtained sintered body was subjected to the lathe turning test and the drilling test. The test methods are as follows.

(1) Lathe Turning Test

Three sintered bodies (ring-shaped, 60 mm (outer diameter)×20 mm (inner diameter)×20 mm (length)) were overlaid on each other, and their side surfaces were turned using a lathe. The turning condition is as follows: the use of a cermet-made lathe turning tool; the turning speed of 100 m/min and 200 m/min; the feed rate of 0.1 mm per cycle; the turning depth of 0.5 mm; and the turning distance of 1000 m. After the test, the flank wear width of the turning tool was measured. Based on the assumption that the tool life corresponds to approximately the wear of 0.25 mm, when the tool life was reached when the turning distance is less than 1000 m, the sample was marked as “1000 m not reached”. It is thus evaluated that the sintered body has more excellent machinability when the flank wear width of the cutting tool is smaller.

(2) Drilling Test

A sintered body (disk-shaped, 60 mm (outer diameter)×10 mm (thickness)) was bored to form a through hole under the conditions of 5,000 rpm in rotational frequency and 750 mm/min in feed rate, using a high speed steel-based drill (2.6 mm in diameter). During this, the thrust component as the cutting resistance in drilling was measured using a tool dynamometer. It is evaluated that the sintered body has more excellent machinability when the thrust component is smaller.

Table 3 shows each of the obtained results.

TABLE 3 Lathe turning result Turning speed Turning speed Drilling test result Sintered Mixed 100 m/min 200 m/min Cutting resistance body powder Flank wear Flank wear Thrust component No. symbol (mm) (mm) (N) Remarks 1 M 1 0.07 0.09 241 Example 2 M 2 0.06 0.06 230 Example 3 M 3 0.08 0.09 250 Example 4 M 4 0.07 0.05 234 Example 5 M 5 0.07 0.08 244 Example 6 M 6 0.08 0.07 255 Example 7 M 7 0.06 0.05 226 Example 8 M 8 0.09 0.08 253 Example 9 M 9 0.08 0.08 249 Example 10 M 10 0.11 0.10 261 Example 11 M 11 0.10 0.09 254 Example 12 M 12 0.09 0.07 250 Example 13 M 13 0.08 0.07 238 Example 14 M 14 0.09 0.08 255 Example 15 M 15 1000 m not 1000 m not 301 Comparative reached reached Example 16 M 16 1000 m not 1000 m not 297 Comparative reached reached Example 17 M 17 0.21 0.23 290 Comparative Example 18 M 18 0.18 0.19 289 Comparative Example 19 M 19 0.17 0.10 275 Comparative Example 20 M 20 0.18 0.09 286 Comparative Example 21 M 21 0.20 0.12 281 Comparative Example 22 M 22 1000 m not 1000 m not 300 Comparative reached reached Example 23 M 23 1000 m not 1000 m not 322 Comparative reached reached Example 24 M 24 1000 m not 1000 m not 312 Comparative reached reached Example 25 M 25 0.22 0.12 289 Comparative Example 26 M 26 1000 m not 1000 m not 294 Comparative reached reached Example 27 M 27 0.06 0.08 224 Example 28 M 28 0.07 0.07 220 Example 29 M 29 0.07 0.06 223 Example 30 M 30 0.07 0.10 233 Example

As shown in Table 3, our Examples all had small flank wear width of the cutting tool, which indicates excellent lathe machinability. Moreover, our Examples had low thrust component in drill boring, and thus were sintered bodies having excellent drill machinability, too. On the other hand, Comparative Examples outside our range especially had poor drill machinability. 

1.-17. (canceled)
 18. A mixed powder for powder metallurgy obtained by mixing an iron-based powder, an alloying powder, a machinability improvement powder, and a lubricant, wherein the machinability improvement powder comprises crystalline layered alkaline silicate heat-treated at 400° C. to 1100° C., and a mix proportion of the machinability improvement powder is 0.01% to 1.0% by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder.
 19. The mixed powder according to claim 18, wherein the machinability improvement powder further comprises at least one selected from the group consisting of an enstatite powder, a talc powder, a kaolin powder, a mica powder, a granulated slag powder, a levigated clay powder, a magnesium oxide (MgO) powder, and a powder mixture of silica (SiO₂) and magnesium oxide (MgO), in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.
 20. The mixed powder according to claim 19, wherein the machinability improvement powder further comprises an alkali metal salt powder in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.
 21. The mixed powder according to claim 20, wherein the alkali metal salt powder is one or two selected from the group consisting of an alkali carbonate powder and an alkali metal soap.
 22. The mixed powder according to claim 18, wherein the machinability improvement powder further comprises a calcium fluoride powder.
 23. The mixed powder according to claim 18, wherein the machinability improvement powder further comprises one or two selected from the group consisting of a metal boride powder and a metal nitride powder.
 24. The mixed powder according to claim 23, wherein the metal boride powder consists of at least one selected from the group consisting of TiB₂, ZrB₂, and NbB₂, and the metal nitride powder consists of at least one selected from the group consisting of TiN, AlN, and Si₃N₄.
 25. The mixed powder according to claim 18, wherein the machinability improvement powder further comprises at least one selected from the group consisting of an alkali metal sulfate and an alkaline earth metal sulfate, in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.
 26. A method of manufacturing a mixed powder for powder metallurgy by preparing and then mixing an iron-based powder, an alloying powder, a machinability improvement powder, and a lubricant to obtain a mixed powder, wherein the machinability improvement powder comprises crystalline layered alkaline silicate heat-treated at 400° C. to 1100° C., and a mix proportion of the machinability improvement powder is 0.01% to 1.0% by mass in terms of total content of the iron-based powder, the alloying powder, and the machinability improvement powder, and the mixing includes: primary mixing in which a part or whole of the machinability improvement powder and a part of the lubricant are added, as a primary mixture material, to the iron-based powder and the alloying powder and heated to perform mixing while melting at least one type of the lubricant, and a resulting mixture is cooled for solidification; and secondary mixing in which a remaining powder of the machinability improvement powder and the lubricant is added, as a secondary mixture material, to the mixture to perform mixing.
 27. The method according to claim 26, wherein the machinability improvement powder further comprises at least one selected from the group consisting of an enstatite powder, a talc powder, a kaolin powder, a mica powder, a granulated slag powder, a levigated clay powder, a magnesium oxide (MgO) powder, and a powder mixture of silica (SiO₂) and magnesium oxide (MgO), in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.
 28. The method according to claim 27, wherein the machinability improvement powder further comprises an alkali metal salt powder in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.
 29. The method according to claim 28, wherein the alkali metal salt powder is one or two selected from the group consisting of an alkali carbonate powder and an alkali metal soap.
 30. The method according to claim 26, wherein the machinability improvement powder further comprises a calcium fluoride powder.
 31. The method according to claim 26, wherein the machinability improvement powder further comprises one or two selected from the group consisting of a metal boride powder and a metal nitride powder.
 32. The method according to claim 31, wherein the metal boride powder consists of at least one selected from the group consisting of TiB₂, ZrB₂, and NbB₂, and the metal nitride powder consists of at least one selected from the group consisting of TiN, AlN, and Si₃N₄.
 33. The method according to claim 26, wherein the machinability improvement powder further comprises at least one selected from the group consisting of an alkali metal sulfate and an alkaline earth metal sulfate, in an amount of 10% to 80% by mass in terms of total content of the machinability improvement powder.
 34. A method of manufacturing an iron-based powder sintered body, by filling a die with a mixed powder for powder metallurgy manufactured by the method according to claim 26, compression-forming the mixed powder into a compact, and subjecting the compact to a sintering process to obtain a sintered body. 